The present invention relates to a method and to an apparatus for preparing filtered coffee, another heat brewable liquid, or the like using microwave radiation during the heating and filtering.
A traditional coffee maker uses a coffee pot on which a separate container is placed. The container is substantially open at its top while being closed on its bottom by a filter e.g. of filter paper. Powdered coffee to be leached by heated water and then filtered is dumped onto the filter paper. In a heating device, which is similar to a flow heater, cold water is heated. At a low rate of flow, the heated water drips onto the ground coffee, seeps through the ground coffee to be mixed with and become drinkable coffee and finally drips through the filter into the coffee pot.
This method is very time consuming and inefficient, since the water which drips down onto the coffee in this process is again cooled, so that the powdered coffee cannot be sufficiently leached out.
It is also known from espresso machines to force hot water or steam at high pressure through a layer of powdered coffee which is contained between two screen surfaces. But this known apparatus is too expensive for household use and also too dangerous for such use because of the high internal pressure used in the machine. Furthermore, it is possible to use this apparatus to prepare filtered coffee rapidly only if hot water or steam is already available.
A coffee maker has also been proposed (Federal Republic of Germany No. OS 2,829,567), in which microwave radiation is used for the heating of water. The water can be heated faster this way than with some other sources of electric heating using the same power. For this purpose, a microwave pervious container for cold water is provided above the filter, which container is intended to receive the ground coffee, and the container is connected to the filter by a thermostatic valve. During use, the entire apparatus is placed in a microwave oven, which is operated to rapidly cause the water to boil. The boiling temperature causes the thermostatic valve to open, and the boiling water passes to the coffee filled filter, through which it drips.
With this unit, the time required for the preparation of filtered coffee is only insignificantly shortened, as compared with the aforementioned known method. Furthermore, care must be taken that the ground filter coffee is carefully screened off from the microwave radiation rays, since tests have shown that ground coffee burns after only a very short period of microwave radiation and long before the water for pouring onto the coffee boils.
In a known apparatus in which coffee is filtered through a filter paper, it has been found that mocha, which is finely ground coffee to which cold water and sugar are added and boiled together, can be filtered only very slowly.
Another apparatus has been proposed (Federal Republic of Germany No. OS 3,206,803) which is substantially similar to a traditional espresso machine. It includes, seated one on the other, a coffee pot, a filter to receive coffee which is closed off by two screening surfaces, and a water container. The water container is pervious to microwave radiation and the filter is developed so that substantially no liquid can pass through the filter under atmospheric or ambient pressure.
The known apparatus is placed in a microwave oven in which the microwave radiation brings the water in the water container to a boil, while the coffee remains screened off. The formation of steam develops such a high pressure in the water container (up to 3.45 bar) that the water is forced through the filter with this pressure.
The use of paper filters is not possible in such an apparatus since, depending on the type of filter paper used, an uncontrolled increase in pressure can take place within the water container.